Letters are being sent to 1.5 million people most at risk of coronavirus, urging them to stay at home.
Those most at risk will receive letters or text messages strongly advising them not to go out for 12 weeks to protect themselves, the government said.
It comes as the PM asked the UK not to visit loved ones on Mother’s Day, and follow social distancing guidelines.
Mayor of London Sadiq Khan asked people to heed the advice, saying “do it for loved ones who will die if you don’t”.
The number of people who have died in the UK with coronavirus rose to 243 on Sunday, as cases topped 5,000.
Boris Johnson has called on the public to join a “collective national effort” and follow social distancing guidance, warning the NHS could be “overwhelmed”.
At-risk people include those who have received organ transplants, those living with severe respiratory conditions such as cystic fibrosis or those who have specific cancers, such as blood or bone marrow.